[R] Handle UTF-8 paths on Windows (#9448)

R-package/R/xgb.Booster.R

@@ -12,7 +12,7 @@ xgb.Booster.handle <- function(params = list(), cachelist = list(),
       ## A filename
       handle <- .Call(XGBoosterCreate_R, cachelist)
       modelfile <- path.expand(modelfile)
-      .Call(XGBoosterLoadModel_R, handle, modelfile[1])
+      .Call(XGBoosterLoadModel_R, handle, enc2utf8(modelfile[1]))
       class(handle) <- "xgb.Booster.handle"
       if (length(params) > 0) {
         xgb.parameters(handle) <- params

R-package/R/xgb.save.R

@@ -43,6 +43,6 @@ xgb.save <- function(model, fname) {
   }
   model <- xgb.Booster.complete(model, saveraw = FALSE)
   fname <- path.expand(fname)
-  .Call(XGBoosterSaveModel_R, model$handle, fname[1])
+  .Call(XGBoosterSaveModel_R, model$handle, enc2utf8(fname[1]))
   return(TRUE)
 }